Effects of Selective Photon Exclusion on Neutrino Oscillation Data
Nandini Vinta, University of Arkansas, Physics Major
Mentored by Dr. Glenn Horton-Smith
Neutrinos are widely considered by scientists to be the strongest lead for explaining baryon asymmetry and for the more general goal of updating the Standard Model. Baryogenesis refers to the theoretical processes that generated an excess of matter over antimatter in the early universe, and neutrinos might explain this phenomenon. Neutrinos are the most abundant particles in the universe and come in three flavors: electron, muon, and tau neutrinos. One of the properties that made neutrinos particularly favorable for this research is called neutrino flavor oscillation. Flavor oscillations are a phenomenon where neutrinos change from between flavors as they travel through space, and are more likely to occur the farther they are allowed to travel.
Neutrino experiments detect particles resulting from interactions between neutrinos and matter since as neutral particles they are virtually undetectable. Previous experiments include MicroBooNE, MiniBooNE, and ProtoDUNE. The Deep Underground Neutrino Experiment (DUNE) is an experiment currently under construction that aims to improve on its predecessors.
Fig. 1.
The experiment is made up of two parts: It starts at the particle accelerator at Fermilab, where protons are accelerated and collide with a target, producing pions which then decay into muon flavor neutrinos. Those neutrinos are shot in a beam to the Sanford Underground Research Facility (SURF) two states over. At SURF is a Liquid Argon Time Projection Chamber (LArTPC), which is essentially a large capacitor filled with Argon atoms and held in a uniform induced electric field. A small percentage of the neutrinos interact with the argon nuclei, producing a variety of particles, including electrons and muons, which are both charged meaning they’re detectable in an electric field.
My research is focused on a specific interaction that occurs in the LArTPC called a Charged Current interaction:
Fig. 2.
The procedure I'm using involves a software called GENIE, which stands for Generates Events for Neutrino Interaction Experiments. GENIE is a Monte Carlo event generator, which means it uses random sampling to simulate the real-life interactions of particles.
In my experimental procedure, I used GENIE to generate neutrino events. I generated 470,000 electron neutrino events for every 0.1 GeV energy level from 0.4 to 5.0 GeV, resulting in 10,000 events for each energy level. I'm considering a high-energy neutrino event to have neutrino energy above 3.0 GeV and a low-energy event to be less than or equal to 3.0 GeV.
The specific focus of my research is on charged current events that produce photons. There are two types of systems that could theoretically produce these photons, but only one is reflected in GENIE event generation, and that is the hadronic systems producing photons:
Fig. 3.
Fig 4.
The above graphs show a pion decaying into two photons or a delta resonance where a positive delta particle decays into a proton which radiates a photon. The other type, which is not yet reflected in GENIE event generation, are the leptonic systems producing photons, in which a photon radiates from either the muon or electron:
Fig. 5.
In previous experiments like MicroBooNE, researchers decided to exclude this photon from calculations for simplicity. Since GENIE runs on previous data, it also does not consider these photons when generating events.
The selective exclusion of these photons could potentially be skewing data. Since the way data is collected in these experiments is by calculating the amount of charged particles and then using that to predict the amount of muon neutrinos and electron neutrinos, excluding this photon means its energy is attributed to the charged particles. This causes under-reporting of the number of electrons and therefore electron neutrinos, leading to a misrepresentation of the probability of neutrino oscillation.
Fig. 6.
Figure 6, shown above, are graphs from my generated data that are plotted over Q2, which is the momentum transfer that we see coming from the neutrino and going to the produced particles. The top graph shows the event counts of all neutrino events, with the blue and green lines representing low-energy and high-energy neutrinos, respectively. These graphs are virtually identical. Below is Figure 7 which shows the same graphs filtered to only show events that produced photons. Again, they are almost identical.
Fig. 7.
The Figure 8 shows graphs from a paper that influenced my research. The graphs show only events that produced photons, including those from leptonic events, which GENIE is currently incapable of producing. These graphs plot momentum transfer versus the ratio between events with photons and total events, essentially the percentage of events that produced a photon. All of these graphs exhibit a fairly linear or slightly downward trend.
Fig. 8.
Fig. 9.
In contrast, my recreation of these graphs (Figure 9) using my generated data shows an increase, peaking at higher energy levels. This discrepancy suggests that the exclusion of leptonic photons is causing a significant enough error in the data, potentially leading researchers to incorrect conclusions about the probability of neutrino oscillation.
Acknowledgements
I would like to thank the following people and organizations: Glenn Horton-Smith, Tim Bolton, Loren Greenman, Bret Flanders, Kim Coy, Kansas State University, and the National Science Foundation. This material is based upon work supported by the National Science Foundation under Grant No. 2244539. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
References
ENIE Physics & User Manual
Theory of QED Radiative Corrections to Neutrino Scattering at Accelerator Energies
QED Radiative Corrections for Accelerator Neutrinos
Search for Neutrino-Induced Neutral Current ∆ Radiative Decay in MicroBooNE and a First Test of the MiniBooNE Low Energy Excess Under a Single-Photon Hypothesis